An objective of the technology in this application is to improve transmission throughput over a radio channel and at the same time maintain a desired transmission error target, like bit or block error rate, by adjusting the channel quality estimate as reported by the receiver. A problem is that the channel quality estimate provided by the receiver often is not accurate, i.e., the radio channel is really better or worse than the channel quality estimate, which means that the response by the transmitter is not optimal, i.e., it overestimates the channel quality resulting in too many errors at the receiver or it underestimates the channel resulting in underutilized transmission capacity.
FIG. 1 illustrates an example cellular radio communications system containing three cells C1, C2, and C3 for purposes of illustration. Each cell has its own radio base station BS1, BS2, and BS3, respectively, for purposes of illustration. In cell C2, a mobile radio terminal, referred to as user equipment 1 (UE1), is receiving a desired communication from its base station BS2. In addition, UE1 is perceiving additional transmissions not targeting UE1 as interference with respect to the desired downlink communication from the base station BS2 to UE1. The interfering signals may be, for example, transmissions from adjacent cell base stations BS1 and BS2, transmissions from UEs in the same cell C2, i.e., UE3, or transmissions from UEs in adjacent cells, i.e., UE2. All of these interfering signals can adversely affect the ability of UE1 to accurately estimate the radio channel quality from base station BS2 to UE1. The inventor recognized the possibility to compensate for one type of interference that is overlooked in the channel quality detection process, namely, interference caused by desired downlink transmissions from base station BS2 to UE1.
Many high speed wireless data systems employ a method of feedback to allow the sending node to make an estimate of the channel quality and based on this make an estimate on how much data can be sent to a user given the power available and the channel quality as reported by the user. Consider as an example, the high speed-downlink shared channel (HS-DSCH) in Wideband Code Division Multiple Access (WCDMA) systems like the Universal Mobile Telecommunications System (UMTS), a channel quality indicator (CQI) is reported by the user equipment (UE) to the UMTS Terrestrial Radio Access Network (UTRAN). The CQI information is contained in the last two slots of the High Speed Dedicated Control Channel (HS-DPCCH) sub frame. These CQI reports are based on measurements on the Common Pilot Channel (CPICH) transmitted by the base station and are reported by the UE at a predefined rate defined by the UTRAN. The CQI in this example represents the instantaneous channel conditions in a predefined 3-time slot interval ending one time slot prior to the CQI transmission.
The accuracy of this CQI estimate has a major impact on the Hybrid Automatic Retransmission Request (HARQ) Block Error Ratio or Rate (BLER) in the UMTS system because a deviation from the actual conditions will either lead to an over-estimation or under-estimation of the CPICH and HS-DSCH conditions, and more generally channel conditions, by the base station transmitter as explained more generally above. The inventor recognized that there are a number of sources that affect the accuracy of the CQI including CQI reporting delay, inter-cell interference, and intra-cell interference. Of particular concern to the inventor is the effect on the CQI report accuracy caused by HS-DSCH transmissions occurring in the cell serving the reporting UE.
In addition to the interference caused by the downlink transmission dedicated to one UE, a similar effect occurs when data is sent to more than one UE in a given transmission time interval (TTI). In WCDMA systems, multiple user transmission by the base station is accomplished using a technique called code multiplexing. The HS-DSCH transmission scheduler shares the radio resources available for transmission over the HS-DSCH by allocating the available spreading codes and transmission power for the HS-DSCH to multiple UEs. The base station uses the CQI from each UE to help determine how much data to send to each UE over the HS-DSCH. So the accuracy of the reported CQIs is of major importance to control BLER and to make efficient use of the HS-DSCH resources.
Currently, the HS-DSCH scheduler does not take into account if the CQI as reported from a UE was determined during a TTI when there was HS-DSCH transmission in the cell serving the reporting UE. Since the radio conditions in a cell are always less than perfectly orthogonal, such a coincidental transmission may well be detected as interference by the UE detecting the channel quality of the pilot transmission in the same base station cell as the HS-DSCH. Nor is the multiplexing transmission to multiple UEs in a code multiplexed implementation taken into account.
Another issue that should be accounted for is the change in transmission status that may occur from the time the UE estimated and reported the CQI until the transmitting node sends data to the UE. For example, if the HS-DSCH scheduler is planning to transmit during an upcoming transmission time interval (TTI), it should take into account both the actual interference conditions that prevailed when the CQI was measured by the UE and an assessment of the amount of interference the planned transmission will cause to the UE due to less than perfectly orthogonal radio conditions. Thus, if there is a difference in the transmission power on the HS-DSCH between the time the UE measured the CQI and at the time of transmission, then the channel conditions will either be over-estimated or under-estimated if not corrected. If the HS-DSCH transmission power is increased at the planned time of transmission, then there is a risk that the transmission will not be correctly decoded by the UE because the interference has increased as compared to the time the UE sent the CQI report. Conversely, if the HS-DSCH transmission power decreases, then the scheduler will overestimate the interference effect and allocate too much power to the HS-DSCH transmission (or choose a smaller transport block than necessary) leading to an inefficient use of the HS-DSCH channel.